One of the first successful fabrications of a continuously operating double-heterojunction (DH) laser at room temperature was reported by I. Hayashi et al. in the Applied Physics Letters of 1970, Volume 17, Pages 109+. Such a laser is comprised of an n-type GaAs substrate and four epitaxial layers comprised of an n-type Ga.sub.(1.sub.-x) Al.sub.x As layer followed by successively built up layers of p-type GaAs, p-type Ga.sub.(1.sub.-x) Al.sub.x As and p-type GaAs. The end product is a single GaAs region surrounded by two heterojunctions, namely, a p-p heterojunction and a p-n heterojunction. When provided with an appropriate resonator, the DH laser exhibits lower thresholds at higher temperatures than are attainable by a single heterojunction laser.
The early DH laser had the disadvantage that the current that produced the necessary carrier injection across a junction for the ultimate light-producing recombination step took place across a relatively large area of the junction. This makes control over the modes of oscillation and removal of the heat from the junction difficult. It was found necessary to fabricate a DH laser so that the active region of the laser was narrowed, confining the carrier injection region to a narrow stripe. Such confinement of the current, hence carrier injection region, to this narrow stripe would facilitate the removal of heat from the junction region and provide laser operation at low threshold currents.
One well known way of obtaining such current confinement is to etch away areas on both sides of a narrow stripe of the active region of the DH laser so that current that produces lasing will only pass through this narrowed region. The portion of the multilayered DH laser that is unetched stands higher than the remaining portion of the laser very much like a mesa rock formation, hence the term "mesa etched."
In another article entitled "Continuous Operation of GaAs-Ga.sub.(1.sub.-x) Al.sub.x As Double-Heterostructure Lasers with 30.degree.C Half-Lives Exceeding 1000 Hours," by R. L. Hartman et al. which appeared in Applied Physics Letters, Vol. 23, No. 4, Aug. 15, 1973, pp. 181-183, the narrowed region of lasing or current confinement is attained by bombarding both sides of a chosen narrow strip of the active region with protons. Such proton bombardment can achieve a lasing stripe width of only 13 microns in that the proton bombarded regions becomes so highly resistive that carrier-injection current can only traverse the unbombarded regions.
It has been found that where proton bombardment is used to attain the high resistivity, no current flow, regions, the bombarded regions are unstable when heated about 350.degree.C. Moreover, the regions adjacent to the active region are damaged, causing increased optical loss in the narrow stripe regions. Additionally, it has been observed that the crystalline damage caused by proton bombardment, close to the active region of the DH laser, will favor and enhance the unwanted diffusion of impurities into that active region. It is also believed that the dark lines that occur in degrading DH lasers may be due to lack of crystalline perfection, and proton bombardment produces such crystalline imperfection.
Where mesa etch is relied upon to achieve current confinement through the DH laser, the end product is a non-planar device having exposed junction edges. Such non-planarity adds to the difficulty of mounting the DH laser to a heat sink, and the exposed edges require additional fabrication steps to passivate.
Two other means have been utilized to obtain the desired current confinement in DH lasers. In one means the electrical contact on the p-type GaAs top layer is made through a conventional oxide mask in such a fashion that the electrical contact does not cover the entire area but only a stripe running from one cleaved face to the other. Depending on the electrical resistivity of the top layers, this has the effect of causing the current flow to occur predominantly in the region below the stripe, although some spreading occurs.
Current confinement to stripes has also been achieved by others by growing the top two layers n-type rather than p-type and diffusing a p-type dopant in the form of a stripe in the region where current flow is desired.